1. Field of the Invention
The present invention relates to an acoustic resonator device, which includes plural acoustic resonators having different resonance frequencies on an identical substrate, and a manufacturing method for the acoustic resonator device. For example, the invention relates to a thin-film acoustic resonator device having two resonance frequencies that are far different from each other. In addition, the invention relates to a filter device that uses an acoustic resonator, and a communication apparatus.
2. Related Art of the Invention
Components incorporated in a communication apparatus are required to be further reduced in a size and a weight. For example, a filter used in a mobile communication apparatus is required to be small in a size and adjusted precisely in frequency characteristics. In recent years, a mobile communication apparatus of a dual mode applicable to plural systems has been mainly used. Accordingly, a filter is also required to be adapted to dual mode having plural different frequencies.
Such a filter is constituted by combining plural thin-film acoustic resonators with a ladder type network.
FIG. 7(a) is a schematic sectional view of a conventional thin-film acoustic resonator. A thin-film acoustic resonator 10 includes a piezoelectric layer 1 and an upper electrode layer 2 and a lower electrode layer 3, which are provided above and below the piezoelectric layer 1, and also includes a substrate 5 below the lower electrode layer 3.
A cavity 4 is provided in the substrate 5 below the lower electrode layer 3, whereby free oscillation of the thin-film resonator 10 is secured and the thin-film resonator 10 operates as an acoustic resonator.
FIG. 7(b) is a schematic perspective view for explaining an operation of the thin-film acoustic resonator 10. When an electric field is applied between the upper electrode layer 2 and the lower electrode layer 3, electric energy is converted into mechanical energy in the piezoelectric layer 1. The thin-film acoustic resonator 10 uses resonance oscillation in a thickness direction of the piezoelectric layer 1 and operates with resonance of a frequency at which the thickness of the piezoelectric layer 1 is equal to λ/2.
Other than the thin-film acoustic resonator having the cavity 4 as shown in FIG. 7, there is known a thin-film acoustic resonator using an acoustic mirror instead of this cavity 4. As such a thin-film acoustic resonator, there are known a λ/2 mode acoustic resonator resonating at half an acoustic wavelength and a λ/4 mode acoustic resonator resonating at a quarter of the acoustic wavelength.
FIG. 8(a) is a sectional view of the λ/2 mode acoustic resonator resonating at half an acoustic wavelength that uses the acoustic mirror (e.g., see Japanese Patent Application Laid-Open No. 9-199978 (e.g., pages 3 to 4, FIG. 1)).
An acoustic mirror 12 is formed on a substrate 11, and a lower electrode layer 15, a piezoelectric layer 16, and an upper electrode layer 17 are formed on the acoustic mirror 12. A thin-film acoustic resonator 18 includes these layers as well as the substrate 11 and the acoustic mirror 12.
In the acoustic mirror 12, a low acoustic impedance layer 13 and a high acoustic impedance layer 14 are stacked on the substrate 11 alternately such that an uppermost layer is the low acoustic impedance layer 13.
The piezoelectric layer 16 has a thickness half of the acoustic wavelength of the resonance frequency. In addition, the respective low acoustic impedance layers 13 and the respective high acoustic impedance layers 14 have a thickness a quarter of the acoustic wavelength of the resonance frequency. The low acoustic impedance layer 13, which is the uppermost layer of the acoustic mirror 12, is in contact with the lower electrode layer 15.
When an electric field is applied between the upper electrode layer 17 and the lower electrode layer 15, the thin-film acoustic resonator 18 uses resonance oscillation in a thickness direction of the piezoelectric layer 16 and resonates at frequency at which the thickness of the piezoelectric layer 16 is equal to λ/2. This resonance oscillation in the thickness direction is confined in the thin-film acoustic resonator 18 by providing the acoustic mirror 12.
FIG. 8(b) is a sectional view of a λ/4 mode acoustic resonator that resonates at a quarter of the acoustic wavelength that uses the acoustic mirror (e.g., see Japanese Patent Application Laid-Open No 6-295181 (e.g., pages 3 to 7, FIG. 2)).
As in the λ/2 mode acoustic resonator shown in FIG. 8(a), an acoustic mirror 72 is formed on the substrate 11, and the lower electrode layer 15, the piezoelectric layer 76, and the upper electrode layer 17 are formed on the acoustic mirror 72. A thin-film acoustic resonator 78 includes these layers as well as the substrate 11 and the acoustic mirror 72.
In the acoustic mirror 72, the high acoustic impedance layer 14 and the low acoustic impedance layer 13 are stacked on the substrate 11 alternately such that an uppermost layer thereof is the high acoustic impedance layer 14. The λ/4 mode acoustic resonator is different from the λ/2 mode acoustic resonator shown in FIG. 8(a) in that the uppermost layer is the high acoustic impedance layer 14.
The piezoelectric layer 76 has a thickness a quarter of the acoustic wavelength of the resonance frequency. The respective low acoustic impedance layers 13 and the high acoustic impedance layer 14 have a thickness a quarter of the acoustic wavelength of the resonance frequency. The high acoustic impedance layer 14, which is the uppermost layer of the acoustic mirror 72, is in contact with the lower electrode layer 15.
When an electric field is applied between the upper electrode layer 17 and the lower electrode layer 15, the thin-film acoustic resonator 78 uses resonance oscillation in a direction of thickness of the piezoelectric layer 76 and resonates at a frequency at which the thickness of the piezoelectric layer 76 is equal to λ/4.
In addition, for example, Japanese Patent Application Laid-Open No. 2002-268645 (e.g., pages 3 to 6, FIG. 1) discloses a technique for constituting a thin-film acoustic resonator that includes plural kinds of thin-film acoustic resonators, which use an acoustic mirror as shown in FIGS. 8(a) and 8(b), on a substrate with thicknesses of piezoelectric layers changed such that resonance frequencies are slightly different from each other and has different frequencies.
FIG. 9 is a sectional view of the thin-film acoustic resonator device constituted in this way. Filters 1, 2, and 3 are formed on the identical substrate 11.
The filter 1 includes a resonator 18a. The resonator 18a includes an upper electrode layer 17a, a piezoelectric layer 161, a lower electrode layer 15a, the acoustic mirror 12, and a substrate 11 below the acoustic mirror 12.
The filter 2 includes resonators 18b and 18c. The resonator 18b includes an upper electrode layer 17b, a piezoelectric layer 162, a lower electrode layer 15b, the acoustic mirror 12, and the substrate 11 below the acoustic mirror 12. The resonator 18c includes the piezoelectric layer 162 having the same thickness as the piezoelectric layer 162 of the resonator 18b, an upper electrode layer 17c, a lower electrode layer 15c, the acoustic mirror 12, and the substrate 11 below the acoustic mirror 12.
The filter 3 includes resonators 18d and 18e. The resonator 18d includes an upper electrode layer 17d, a piezoelectric layer 163, a lower electrode layer 15d, the acoustic mirror 12, and the substrate 11 below the acoustic mirror 12. The resonator 18e includes the piezoelectric layer 163 having the same thickness as the piezoelectric layer 163 of the resonator 18d, an upper electrode layer 17e, a lower electrode layer 15e, the acoustic mirror 12, and the substrate 11 below the acoustic mirror 12.
The thicknesses of the piezoelectric layers 161, 162, and 163 are different from each other. In addition, the thicknesses of the upper electrode layers 17a, 17b, 17c, 17d, and 17e are different from each other and the thicknesses of the lower electrode layers 15a, 15b, 15c, 15d, and 15e are different from each other. By setting different thicknesses for the respective layers, a thin-film acoustic resonator device including filters having different resonance frequencies is realized. The entire disclosure of Japanese Patent Application Laid-Open No. 9-199978 (e.g., pages 3 to 4, FIG. 1)), Japanese Patent Application Laid-Open No 6-295181 (e.g., pages 3 to 7, FIG. 2)), and Japanese Patent Application Laid-Open No. 2002-268645 (e.g., pages 3 to 6, FIG. 1) are incorporated herein by reference in their entireties.
However, in the thin-film acoustic resonator device of the structure of the conventional technique shown in FIG. 9, the piezoelectric layers 161, 162, and 163 of the respective resonators have to be formed such that thicknesses thereof are different from each other and the lower electrode layers have to be formed such that thicknesses thereof are different in the lower electrode layer 15a, the pair of lower electrode layers 15b and 15c, and the pair of lower electrode layers 15d and 15e, and there are deficiencies as described below. In a thin-film acoustic resonator device, all of respective electrode layers, respective layers constituting an acoustic mirror, and piezoelectric layers are formed by a film formation process. In order to make thicknesses of the piezoelectric layers and the lower electrode layers different from each other by such a process, at least the filters 1, 2, and 3 have to be formed by different steps. However, formation of each filter affects formation of the other filers.
These deficiencies make it difficult to maintain equal performance for all acoustic resonators with resonance frequencies different from each other provided on a substrate.